Cellular environments are dense with proteins and nucleic acids at high concentrations. Results from computer simulations and experiments are presented that explore dynamic associations ranging from clusters to condensates. From atomistic simulations and extensive comparisons with experimental data, cluster formation involving proteins is identified as a major determinant of slowed-down diffusion at high concentration depending on protein interaction strength but also contact life times. Results from coarse-grained simulations with the recently developed COCOMO model explore charge-driven condensation involving different systems ranging from globular proteins and RNA to short peptides and RNA fragment and condensates composed of intrinsically disordered peptides. Finally, recent results form atomistic simulations of condensates are presented to discuss molecular densities and dynamics inside biomolecular condensates.